The invention relates to a system for Connecting and disconnecting a lower end of a marine riser to and from a blow out preventer stack on a subsea wellhead according to the preamble of claim 1.
Drilling of offshore hydrocarbon wells is performed by a drill string arranged in a riser extending from a blow out preventer stack on a wellhead on the sea floor to a drilling vessel. The drilling vessel may be anchored to the sea floor or kept in position by thrusters of a dynamic positioning system. The lower end of the riser is connected to the blow out preventer stack by a riser connector, which includes some type of hydraulically operated gripping members, such as fingers which in a clamping position clamp a flange of the lower end of the riser. The riser connector also includes a lock element, which by means of hydraulic actuators can be moved between a lock position in which the lock element locks the gripping members in the clamping position, and an unlock position in which the gripping members are free to move to a release position, i.e. a position which allows connecting and disconnecting the riser end.
Connectors which may be used for connecting a riser to a wellhead are disclosed in U.S. Pat. No. 4,721,132, U.S. Pat. No. 5,382,056 and U.S. Pat. No. 6,234,252.
In order to allow a transversal movement of the drilling vessel, which may be caused by wind, waves and current, the riser is normally connected to the riser connector via a flexible joint which allows some angular displacement of the riser. To allow a vertical movement of the drilling vessel, the riser is also equipped with a telescopic joint. If the angular displacement of the riser exceeds a maximum acceptable angle, dictated by mechanical limitations of the flexible joint or the telescopic joint, the riser will be disconnected from the blow out preventer stack on the wellhead.
When disconnecting the riser the hydraulic actuators are pressurised to move the lock element to the unlock position. The gripping members are then free to move to the release position, and the riser can be withdrawn and disconnected. For various reasons, e.g. a jamming of the lock element, moving the lock element to the unlock position may require greater forces than moving the lock element to the lock position. For this reason the hydraulic actuators may consist of primary hydraulic actuators able to move the lock element between the lock position and the unlock position, and secondary hydraulic actuators merely able to move the lock element to the unlock position. Thereby greater forces are available for moving the lock element to the unlock position then for moving the lock element to the lock position.
Hydraulic circuitry which pressurise the hydraulic actuators may for various reasons fail. Reasons for failure include malfunctioning of valves, clogging or rupture of hydraulic lines or jamming of the hydraulic actuators. In order to increase the reliability of the hydraulic circuitry the circuitry may comprise a hydraulic primary unlock circuit for actuating the primary actuators and a hydraulic secondary unlock circuit for actuating the secondary actuators. This a proven design which is in use with many riser connectors. There is, however, a wish to further increase the reliability of the hydraulic circuitry, but in order to gain acceptance in the market, a system with increased reliability should also include the proven design comprising the primary aid secondary unlock circuits.
The objective of the invention is therefore to provide a system for connecting and disconnecting a lower end of a marine riser to and from a blow out preventer stack on a subsea wellhead, which system shall comprise a highly reliable backup system for disconnecting the riser. A further objective is that the system shall combine the proven design comprising the primary and secondary hydraulic unlock circuits with the backup system.
The objectives are achieved by a system according to the claims.
The invention then provides a system for connecting and disconnecting a lower end of a marine riser to and from a blow out preventer stack on a subsea wellhead, comprising:
a riser connector comprising:
gripping members movable between a clamping position in which they clamp the riser end and a release position in which the riser end is released,
a lock element movable between a lock position in which the lock element lock the gripping members in the clamping position and an unlock position in which the gripping members are free to move to their release position,
primary hydraulic actuators able to move the lock element between the lock position and the unlock position,
secondary hydraulic actuators merely able to move the lock element to the unlock position, and
hydraulic circuitry comprising:
a hydraulic lock circuit for actuating the primary actuators to move the lock element to the lock position and move the secondary actuators to a position from which they can move the lock element to the unlock position,
a hydraulic primary unlock circuit for actuating the primary actuators to move the lock element to the unlock position,
a hydraulic secondary unlock circuit for actuating the secondary actuators to move the lock element to the unlock position.
According to the invention,
the primary unlock circuit comprises a first primary unlock circuit for actuating first primary actuators and a second primary unlock circuit for actuating second primary actuators, and the system further comprises:
a hydraulic backup unlock circuit for actuating the primary and secondary actuators to move the lock element to the unlock position, the backup unlock circuit comprises a source for pressurised hydraulic fluid and a triple flow divider for dividing fluid flow from the source into one flow for actuating the first primary actuators, one flow for actuating the second primary actuators and one flow for actuating the secondary actuators.
The invention thereby provides a hydraulic backup unlock circuit with three independent flows for actuating the actuators which move the lock element to the unlock position. A rupture in a conduit for one of these independent flows will result in that the actuators which are supplied from this conduit will fail in moving the lock element to the unlock position, while the remaining actuators will maintain their ability to move the lock element to the unlock position. It is thereby provided a highly reliable backup system for disconnecting the riser.
Further, by dividing the primary unlock circuit into the first primary unlock circuit and the second primary unlock circuit, the two unlock circuits according to proven design, namely the primary and secondary unlock circuits, are combined with the backup system.